Tests for library

[michalchudziak]

• Integrate jest and write some simple unit tests.
• Integrate detox and write some simple e2e tests.
• Use JUnit / XCtest to test native code.

[alpharius-ck]

would be working on this one

[artus9033]

We've already got unit tests covered. E2E tests are tracked here:

• iOS: ci: run E2E suites in demo apps #352
• Android: ci: run E2E suites in demo apps (Android) #381